The invention relates to installations or ovens used for performing heat treatment, and in which the gas used in the treatment is preheated prior to being introduced into the treatment chamber of the installation. Such installations are used in particular for performing thermochemical treatments, such as carburizing parts or densifying porous substrates by chemical vapor infiltration.
A field of application of the invention is that of making parts out of thermostructural composite material, i.e. out of composite material that possesses both mechanical properties that make it suitable for constituting structural parts and also the ability to conserve those properties up to high temperatures. Typical examples of thermostructural composite materials are carbon/carbon (C/C) composites have a reinforcing fabric of carbon fibers densified by a matrix of pyrolytic carbon, and ceramic matrix composites (CMC) having a reinforcing fabric of refractory fibers (carbon or ceramic) densified by a ceramic matrix.
A well-known method for densifying porous substrates in order to make C/C or CMC composite parts is chemical vapor infiltration (CVI). The substrates for densifying are placed in a loading zone of an installation in Which they are heated. A reactive gas containing one or more gaseous precursors of the material constituting the matrix is introduced into the oven. The temperature and the pressure inside the installation are adjusted so as to enable the reactive gas to diffuse within the pores of the substrates and deposit therein the material constituting the matrix as a result of one or more of the components of the reactive gas decomposing or as a result of a reaction between a plurality of components, these components forming the precursor of the matrix. The process is performed under low pressure so as to encourage the reactive gas to diffuse within the substrates. The temperature of the transformation of the precursor(s) in order to form the matrix material, such as pyrolytic carbon or ceramic, usually lies in the range 900° C. to 1100° C., but this temperature may nevertheless be as high as 2000° C. for a massive deposit of pyrolytic carbon by chemical vapor deposition (CVD).
In order to perform densification that is as uniform as possible in the substrates throughout the loading zone, whether this be measured in terms of increase in density or in terms of the microstructure of the matrix material that is formed, it is necessary for the reactive gas to penetrate into the loading zone at a temperature that is as low as possible and uniform.
Thus, installations conventionally include means for preheating the gas. Such preheater means may be situated at the periphery of the installation, i.e. on the path of the gas before it enters into the treatment enclosure of the installation. Such preheater means increase the complexity and the overall size of the installation.
In order to avoid those drawbacks, it is known to provide the installation with a zone or chamber for preheating the reactive gas that is situated between the inlet for the reactive gas into the installation and the loading zone. Typically, the preheater zone comprises a plurality of perforated trays through which the reactive gas passes.
The gas preheater trays, like the substrates, are heated as a result of being present in the installation. The installation is generally heated by induction or by electrical means such as resistor elements housed in the wall of the installation.
Nevertheless, although the preheater chamber enables the reactive gas to be heated prior to being introduced into the loading zone, it is difficult to control the temperature of the reactive gas in the preheater chamber so that it is uniform radially, in particular in installations of large diameter.
In order to solve that problem, it might be thought that the effectiveness with which the gas is preheated could be increased by enlarging the preheater zone, in particular by increasing its volume vertically, even though that is to the detriment of the volume of the loading zone for an installation of unchanging total volume. Unfortunately, treatments such as chemical vapor infiltration processes require investments that are expensive on an industrial scale, and they take a long time to perform. It is therefore very desirable for installations to have a high level of productivity, regardless of whether they are installations that are already in service or new installations that are yet to be made, and thus for them to have a ratio that is as great as possible between the working volume dedicated to being loaded with substrates or parts for treatment, and the volume that is dedicated to heating the reactive gas.